Engine exhaust often contains incomplete combustion compounds such as hydrocarbons, carbon monoxide (CO) and nitrogen oxides (NOx). These compounds have to be removed from engine exhaust for air pollution control and to satisfy various government regulations. There are various systems that have been created for various types of engines and fuel configurations to address the challenging emission control problems. These include three way catalysts, close-coupled catalysts, as well as filters or catalyzed filters. Most of these catalysts or combined catalyst systems are based on precious metals, including Pt, Pd and Rh. Although these noble metal catalysts are effective for mobile emission control and have been commercialized in industry, precious metals are extremely expensive. This high cost remains a critical factor for a wide variety applications of these catalysts. There is a constant need of alternative, cheaper catalysts for the effective removal of HC, CO and NOx compounds from mobile emission sources in order to meet increasingly stringent regulations.
One possible alternative has been the use of base metals. Base metals are abundant and much less costly than the precious metals. Several attempts have been made to develop base metal based catalysts for emission control. However, each of these attempts has been fraught with problems. For example, some monolith catalysts have been made that result in the formation of AB2O4 and perovskite type crystal ABO3. However, formation of a perovskite structure significantly reduces the catalyst surface area. In other attempts, Cr has been used. However, Cr is highly toxic. Base metal formulations containing both Zn and Cr are likely to lead to catalyst deactivation as a result of Zn loss and regulatory barrier, due to toxicity of Cr. Other base metal catalysts have simply not been able to achieve acceptable levels of pollutant reduction.
Additionally, one area where such a catalyst would be extremely beneficial is with the treatment of exhaust from motorcycles. Motorcycles are more affordable alternatives to automobiles, and as such have become very popular. This is particularly true for some areas of the world, especially in parts of Asia where motorcycles have become extremely popular, and outnumber cars. Thus, there is still a need for an affordable, yet effective, catalyst. In particular, there is a need for such a catalyst for mobile source applications, especially for motorcycles.